This invention relates generally to microwave guides and component parts therefore. More particularly, this invention relates to an improved microwave waveguide seal which maintains consistent and uniform electrical characteristics through a range of temperature, pressure and chemical environments.
While the present invention will be described in connection with a microwave level gauge for use in a container such as a rail car, it will be appreciated that the microwave waveguide and components therefore may be useful for a plurality of other applications including any application requiring waveguides and microwave components including, but not limited to, chemical processing vessels, waste storage tanks, ships and barges.
U.S. application Ser. No. 07/729,457 ('457) filed Jul. 17, 1991 now U.S. Pat. No. 5,305,237 (all of the contents of which are fully incorporated herein by reference thereto) discloses a method and apparatus for monitoring the level of flowable material contained in a vessel such as a tank. The level monitoring system of the '457 application includes a microwave seal assembly, a microwave transceiver assembly, a controller/processor assembly and a computer. The microwave transceiver assembly is adapted to the tank and the controller/processor assembly and the computer may be located locally or remotely. The seal assembly is permanently affixed to the tank in sealing relationship with an opening formed in a fitting that communicates with an upper portion of the tank. The transceiver assembly emits microwave signals through the seal toward the surface of the material and receives microwave signals reflected from the surface through the seal.
In a preferred embodiment of the seal assembly disclosed in the '457 application, a wave guide member is provided that extends into the tank and has an open upper end portion that is in facing relationship with the seal. The lower end portion of the wave guide member extends towards the bottom of the tank and is in fluid communication therewith. The seal assembly and the wave guide member are preferably welded together and secured as an assembly to the tank by a connecting ring that is welded to the wave guide member. In accordance with an alternative embodiment, the seal assembly communicates with the interior of the tank through a ball valve member. Other embodiments include the use of various types of antenna inside the tank.
Seal assemblies of the type described in the '457 application suffer from certain drawbacks and deficiencies. Serious problems have been encountered due to the need for sealing under high pressure and often in hostile environments. For example, it is problematic to form a gas tight seal in a dielectric filled waveguide. Conventional sealing techniques tend to disturb the electrical characteristics of the waveguide or are unable to provide the same temperature and chemical capabilities as the dielectric filling material. Similarly, it is difficult to retain a solid dielectric filling material in a waveguide against high pressure without changing the effective electrical diameter of the waveguide. While it is possible to retain a dielectric material in a waveguide by including small retaining features on the dielectric material, this scheme makes assembly complex. Since the features must be kept small so as not to create microwave reflections, it is difficult to design a system to retain high pressures when using a soft dielectric such as PTFE.
Still other problems are encountered in sealing disks or membrane made from PTFE or similarly soft materials for high pressure gas operation over a broad temperature range. Because of the problems of chemical compatibility with the contained material, conventional gaskets or "O" rings cannot be used. Gaskets or "O" rings made from or encapsulated with the same material as the membrane do not provide sufficient resiliency over the required temperature range. Moreover, a conventional PTFE to metal seal is not suitable as a high pressure seal because of the tendency of PTFE to cold flow. Once flow at high temperature relaxes the seal area, the contraction of the PTFE at low temperature can cause seal failure.